Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor that contains a so-called heterojunction between two materials having different band gaps, such as GaN and AlGaN. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises near the interface between the AlGaN barrier layer and the GaN buffer layer. In an HEMT, the 2DEG forms the channel of the device. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas. The two-dimensional carrier gas is intrinsically conductive.
Due to the high electron mobility of the two-dimensional carrier gas in the heterojunction configuration, HEMTs offer high conduction and low losses in comparison to many conventional semiconductor transistor designs. These advantageous conduction characteristics make HEMTs desirable in applications including, but not limited to, use as switches in power supplies and power converters, electric cars, air-conditioners, and in consumer electronics, for example.
In an HEMT, the charge carrier density of the two-dimensional carrier gas that provides the channel of the device is an important performance parameter. For this reason, it is desirable to test the charge carrier density of the two-dimensional carrier gas of HEMT devices under manufacture. Conventionally, this kind of measurement has only been possible on bare semiconductor wafers. That is, it has not been possible up to date to measure the charge carrier density of individual integrated circuits, e.g., discrete HEMTs, during the manufacture of these devices.